Savage 340

Is there any way to estimate the production date of these older 340's?
I picked one up in 30-30 a couple weeks ago and was wondering how old it was.
Receiver has SAVAGE ARMS CORPORATION
CHICOPEE FALLS, MASS U.S.A. SAVAGE 340
PROOF TESTED CAL. 30-30
In front of the barrel nut there is a SP inside a circle and a 6D inside a circle. The rear sight has Dockendorff written on it in cursive. I can't find a serial number anywhere on it.